I've been playing a game called PMOG: Passively Multiplayer Online Game (currently in closed beta, but I have some invitations available; if you want one, leave a comment). It sort of adds a second background layer to the web -- one with a game in it. Players install a Firefox extension and accumulate points as they visit different websites during the course of their daily surfing. You can lay mines as traps for other players on sites (in the first screenshot, I tripped a mine laid on wikipedia.org by another player; don't worry, my browser was wearing armor so I'm fine), or leave portals (links) from one site to another.
What I'm interested in here is the missions feature. If you're not familiar with MMOGs, a common gameplay feature is the quest-giver or mission giver, who offer tasks for your character to complete (in World of Warcraft, they have big yellow question marks over their heads so you can spot them). In PMOG, players leave mission starting points all over the web, and you either happen across missions serendipitously or pick them from a list on the PMOG site. (In this photo I ran across a mission on flickr.com.)
A mission is basically a tour of a series of websites with explanatory text. Since you get PMOG "datapoints" for visiting new sites, you get points as you complete missions -- but the really interesting feature is that it provides a way for one user to lead another through a series of sites and comment on each one. I haven't seen another application that does this sort of thing. My first thought was "Jeez, I wish there was an easy way for us to use this to create tutorials." How cool would it be to add our own commentaries on database sites for our users? Or lead them from one database to another? Or to a research guide?
An application like this allows for commentary to take place without interfering with the actual browsing experience, or requiring any content to be added to the page itself. It makes me think of the help text that appears onscreen during a game play tutorial, which prompts the user to try certain actions but allows actual play to continue. I've been thinking for a while that game tutorials were a help model that libraries should try to emulate. They usually take place in the live game environment, so you don't have to read a manual before starting to actually play; they just add a level of instruction to the live experience.